1. Field of the Invention
The present invention relates to a differential resonance magnetometer. It is used in the relative measurement of weak magnetic fields, e.g. of a few dozen microteslas, i.e. in the measurement of the space and/or time variations of such fields.
The invention has applications in geophysics, mining prospecting, space detection, medicine, etc.
2. Discussion of the Background
The magnetometer according to the invention is of the directional resonance type. These apparatuses are well known and are e.g. described in French patent application FR-A-2 663 751 filed by the present applicant (said application containing bibliographical references on the subject) or in the corresponding European patent application EP-A-463 919, or in the article by D. DURET, M. MOUSSAVI and M. BERANGER entitled "Use of High Performance Electron Spin Resonance Materials for the Design of Scalar and Vectorial Magnetometers", published in IEEE Transactions on Magnetics, vol.27, no. 6, Nov. 1991, pp.5405-5407.
The structure and operation of these apparatuses will briefly be described in certain of their variants with reference to the attached FIGS. 1 to 6.
Firstly, in FIG. 1, it is possible to see a sample 2 containing a material having electron or nuclear spins, a first winding 3 producing a magnetic polarization field Hb directed in direction D, a second winding 4 in the vicinity of the sample 2, a current generator 5 having a regulatable intensity supplying the first winding 3, a high frequency generator 6 connected to a measuring bridge 8, which is in turn connected to a resonant circuit 10 containing the winding 4, a low noise amplifier 16 connected to the measuring bridge 8, a balance mixer 18 receiving on the one hand the voltage supplied by the amplifier 16 and on the other a reference voltage from the high frequency generator 6 and finally a low pass filter 20.
The sample 2 is subject to the field to be measured Hm, as well as to the polarization field Hb. These two fields are not generally colinear. The magnetic field measured by such an apparatus is the sum of Hb and the component of Hm projected in the direction D and which is designated (Hm).sub.D, bearing in mind that Hb is much greater than Hm. These different fields or components are shown in FIG. 2. The total field in the direction D is designated H.
The apparatus functions as follows. The generator 6, connected to the circuit 10 and to its winding 4, is able to excite the resonance of the spins of the sample 2. Its frequency is very accurately fixed (10.sup.-9 to 10.sup.-6). The resonance of the spins occurs when the frequency of of the excitation signal is equal to the LARMOR frequency, conventionally determined by the relation (1/(2.pi.)).gamma.Ho, in which .gamma. is the gyromagnetic ratio of the sample used (in the case of the electron .gamma.=2.pi. 28 GHz/T) and Ho is the value at resonance of the total magnetic field applied.
At the same time the circuit 10 and its winding 4 are able to detect this resonance, the function of the circuit 8 being to separate the excitation and the detection.
The apparatus shown in FIG. 1 detects the passage through the resonance when, the frequency of being fixed, the total field H passes through the value Ho. Thus, FIG. 3 shows the variations of the voltage V1 supplied by the low pass filter 20 when the field H varies and passes through Ho. This curve is of the dispersion type, i.e. antisymmetrical, with a positive part, a cancelling out (for the value Ho corresponding to resonance) and a negative part. The knowledge of Ho makes it possible to rise to (Hm).sub.D if Hb is known.
The apparatus of FIG. 4 is a variant where use is also made of an oscillator 22 and a winding 24 for producing a magnetic field having an audio frequency fm, said field, known as the dither field being superimposed on the polarization field Hb.
Moreover, in the variant of FIG. 4 and at the output of the balanced mixer 18, the low pass filter 20 of FIG. 1 is replaced by a band pass filter 26 centered around the frequency fm. A phase shifter 28 receives the high frequency signal from the generator 6 and supplies a signal of appropriate phase to the balanced mixer 18.
A circuit 30 for synchronous detection at the frequency fm receives on one of its inputs a reference signal from the generator 22. This reference signal has a frequency fm, but its amplitude and phase can be made different from those of the signal supplied by the oscillator 22 to the coil 24. The circuit 30 has another input connected to the output of the filter 26 and finally supplies a voltage Vs.
By fitting at the output of the synchronous detection means 30 an appropriate, not shown, observation means, it is possible to observe the curve of the variations of Vs as a function of the total field H. This curve is shown in FIG. 5. Like that of FIG. 3, it is an antisymmetrical curve with a cancelling out for the value Ho of the field corresponding to the resonance of the spins.
With such apparatuses, a variation of the field to be measured Hm, if it is well below the width of the lines shown in FIGS. 3 and 5, leads to a variation compared with the resonance value and by the appearance of a non-zero voltage (V1 or Vs) at the magnetometer output. This voltage varies substantially linearly as a function of the variation at Ho.
The linearity can be improved by a field feedback obtained by using the voltage V1 (FIG. 3) or the voltage Vs (FIG. 5) as the error signal, by integrating said voltage and by injecting into a feedback coil a current proportional to the integrated voltage. The axis of this feedback coil must be parallel to the direction D of the polarization field.
This can be seen in the diagram of FIG. 6 where, in addition to the means already shown in FIG. 4, there are an integrator 31 and a feedback winding 32. In such an apparatus, the total field is still maintained at the value corresponding to resonance and the integrated error signal constitutes the measurement signal, which appears on the apparatus output 34.
In other words, no matter what the field applied from the outside to the sample, along the direction D the said sample sees the same field, namely that ensuring the resonance of the spins. As the field corresponding to resonance is much larger (more than ten times) than the field having an external origin to be measured, the geometric sum modulus (cf. FIG. 2) is substantially equal to the sum of the field created by the polarization current and directed in accordance with direction D and the projection on said direction of the external field to be measured. In other words, the means 31 and 32 make the apparatus dependent on the resonance, no matter what the field applied from the outside.
In the embodiment illustrated in FIG. 6, it should be also noted that there are three windings, respectively polarization 3, dither 24 and feedback 32, which are assumed to be separate. However, as they all have the same axis, they can be combined into one and the same winding.
The integrator 31 can either directly supply the feedback current, or can supply a voltage, in which case it is necessary to associate a voltage - current converter with it, e.g. in the form of a resistor.
Thus, according to this prior art, known directional resonance magnetometers generally have a sample with spins subject to the field to be measured, means for applying to said sample a magnetic polarization field, means for applying to the sample a radio-frequency field and for exciting the resonance of the spins, means for detecting said resonance, means for producing an anti-symmetrical electrical signal which is cancelled out when the total field applied to the sample assumes the value Ho corresponding to the resonance of the spins and which is positive or negative as a function of whether the total field applied is higher or lower than said particular value (Ho). In the absence of a field to be measured, the apparatus is regulated so that it is at resonance. In the presence of a field to be measured, the feedback field reestablishes the resonance and the value of the integrated error signal (or the feedback current) constitutes the measurement of the magnetic field.
In order to produce a differential magnetometer with the means described hereinbefore, it is necessary to have two such apparatuses at two separate points and form the difference between the signals obtained.
However, the two measurement chains used in these two apparatuses will not generally be perfectly identical, so that on the one hand the difference signal obtained will in reality be due to structural differences of two apparatuses. On the other, as the calculated difference is small compared with the two separately measured quantities, said difference must be very accurately calculated in order to be significant.